1. Field of the Invention
The present invention pertains to a pressure/flow control valve for use in a fluid delivery system, and, in particular, to a pressure/flow control valve whose movement is dampened non-linearly to improve valve response and controllability in the fluid delivery system.
2. Description of the Related Art
There are numerous situations where it is desirable to control the flow of fluid through a conduit. For example, in the medical field, a flow of gas is delivered to a patient by a conventional mechanical ventilator to supplement or replace the patient's own respiration. It is necessary to control the flow and/or pressure of gas delivered to the patient so that proper volume and/or pressure of gas is provided by the ventilator to the patient at the proper time.
Other fluid delivery systems are known for providing a flow of gas to an airway of a patient at an elevated pressure to treat a medical disorder. For example, it is known to use a continuous positive airway pressure (CPAP) device to supply a constant positive pressure to the airway of a patient to treat obstructive sleep apnea (OSA). It is also known to use a fluid delivery system to provide a bi-level positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient's breathing cycle or varies with the patient's effort to increase the comfort to the patient. It is further known to provide an auto-titration positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea or upper airway resistance.
Conventional fluid delivery systems, which include medical ventilators, CPAP, bi-level, and auto-titrating systems (and variants thereof), typically include a pressure generator, for example, a blower, piston, or bellows, that creates a flow of breathing gas at a pressure greater than the ambient atmospheric pressure. A patient circuit delivers the elevated pressure breathing gas to the airway of the patient. Typically, the patient circuit includes a conduit, e.g., a single limb or lumen, having one end coupled to the pressure generator and a patient interface device coupled to the other end of the conduit. The patient interface connects the conduit with the airway of the patient so that the elevated pressure gas flow is delivered to the patient's airway. In a closed system, e.g., when providing ventilation to the patient, a second limb or lumen is provided that couples the patient interface with an exhaust valve. In all of these pressure support therapies and patient ventilation techniques, as well as many not mentioned, it is important to control the pressure or the flow of gas delivered to the patient through the patient circuit.
In a typical fluid delivery system, a valve is provided to control the pressure and/or flow of gas delivered by the pressure generator to the patient. Typically, the valve is provided downstream of the pressure generator in, or associated with, the fluid carrying patient circuit. In some conventional systems, the valve provides a variable sized restriction to the flow of fluid through the conduit. It is also known to divert or exhaust gas from the patient circuit using a variable orifice valve to control the pressure, and, hence, the flow of gas to the patient. The flow restricting and flow diverting approaches have also been used in combination.
A control system is typically provided to control the actuation of the valve or valves. If capable of operating in synchronization with the patient's respiratory efforts, the control system for the ventilator or fluid delivery system includes the ability to monitor the respiratory effort of the patient and control the actuation of the valve in a feed-back fashion based on this monitoring. For example, a pressure sensor and a flow sensor are typically provided to monitor the pressure and flow in the patient circuit or patient interface to detect when the patient attempts to transition between an inspiratory and an expiratory phase of the breathing cycle. When a transition is detected, a controller causes the pressure/flow control valve to open or close accordingly.
It can be appreciated that the valve must be able to react quickly to detected changes in the patient respiration so that a transition in pressure or flow is provided immediately upon detecting the change in respiratory state. In addition, the valve must be able to accurately move to its operating position to ensure that the proper pressure and/or flow is delivered to the patient.
It is known in some conventional pressure/flow control valves to provide feedback information, such as the position, speed, or acceleration of the moving component of the valve relative to the stationary component, to assist in controlling the operation of the valve. However, this type of feedback control increases the complexity of the valve and does not provide maximum response time, as the position of the valve must be continuously monitored and controlled in a feedback fashion, thereby slowing down its actuation time. Therefore, it is desirable to avoid using feedback information in controlling valve movement.
However, it is a challenge to control the position of force-activated actuators in time-variant motion control systems with uncertain system dynamics without utilizing velocity and/or position feedback. In medical ventilators, flow or pressure is regulated by controlling the position (opening area) of a moving member relative to a stationary member in a valve. When the control input to the moving member is force or a force-related parameter, such as a coil current in voice coil valves, accurate positioning of the valve for delivering specific flow or pressure patterns is challenging without velocity and/or position feedback. This task becomes even more challenging in the presence of stick-slip conditions and due to the relatively high acceleration of the moving member, which typically has a small mass. Also, in a pressure generating system, the pattern and rate of valve movement for achieving fixed flow or pressure patterns is dependent on the uncertain, i.e., unmeasured, patient lung dynamics and breathing behavior. These factors and the tight performance requirements for a fluid delivery system, especially a life supporting medical ventilator, demand motion control systems with accurate and fast parameter, i.e., flow and/or pressure regulation.